Preamble design for supporting multiple topologies with visible light communication

ABSTRACT

For use in visible light communication (VLC), methods for synchronization with multiple topology support and for transmitting an extended preamble. The method for synchronization includes transmitting a two-part preamble sequence. The preamble sequence includes one or more repetitions of a fast locking pattern (FLP) configured to be used for clock synchronization, and one or more repetitions of a topology dependent pattern (TDP) configured to be used to distinguish a plurality of VLC topologies. The method for transmitting an extended preamble includes generating an extended preamble and transmitting the extended preamble during a receive or idle mode for maintaining visibility support and for better synchronization performance.

CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY

The present application is related to U.S. Provisional Patent No.61/276,782, filed Sep. 16, 2009, entitled “MULTIPLE PREAMBLES FORSUPPORTING MULTIPLE APPLICATIONS WITH VISIBLE LIGHT COMMUNICATION”.Provisional Patent No. 61/276,782 is assigned to the assignee of thepresent application and is hereby incorporated by reference into thepresent application as if fully set forth herein. The presentapplication hereby claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent No. 61/276,782.

The present application also is related to U.S. Provisional Patent No.61/280,139, filed Oct. 30, 2009, entitled “MULTIPLE PREAMBLES FORSUPPORTING MULTIPLE APPLICATIONS WITH VISIBLE LIGHT COMMUNICATION”.Provisional Patent No. 61/280,139 is assigned to the assignee of thepresent application and is hereby incorporated by reference into thepresent application as if fully set forth herein. The presentapplication hereby claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent No. 61/280,139.

TECHNICAL FIELD OF THE INVENTION

The present application relates generally to visible light communicationand, more specifically, to preamble design and the use of multiplepreambles to support multiple topologies in visible light communication.

BACKGROUND OF THE INVENTION

Visible light communication (VLC) is a new technology for short-rangeoptical wireless communication using visible light in opticallytransparent media. This technology provides access to several hundredterahertz (THz) of unlicensed spectrum. VLC is immune to the problems ofelectromagnetic interference and non-interference associated with radiofrequency (RF) systems. VLC provides an additional level of security byallowing a user to see the transmission of data across the communicationchannel. Another benefit of VLC is that it augments and complementsexisting services (such as illumination, display, indication,decoration, etc.) from existing visible-light infrastructures. A VLCnetwork is any network of two or more devices that engage in VLC.

FIG. 1 depicts the full electromagnetic frequency spectrum, and abreakout of the wavelengths occupied by visible light. The visible lightspectrum extends from approximately 380 to 780 nm in wavelength, whichcorresponds to a frequency range of approximately 400 to 790 THz. Sincethis spectrum is large and can support light sources with multiplecolors, VLC technology can provide a large number of channels forcommunication.

SUMMARY OF THE INVENTION

For use in visible light communication (VLC), a method forsynchronization is provided. The method includes transmitting a two-partpreamble sequence. The preamble sequence includes one or morerepetitions of a fast locking pattern (FLP) configured to be used forclock synchronization, and one or more repetitions of a topologydependent pattern (TDP) configured to be used to distinguish a pluralityof VLC topologies.

For use in visible light communication (VLC), a method for transmittingan extended preamble is provided. The method includes generating anextended preamble and transmitting the extended preamble during areceive or idle mode for maintaining visibility support and for bettersynchronization performance.

For use in visible light communication (VLC), a method fordistinguishing multiple VLC transmissions is provided. The methodincludes receiving a first preamble associated with a first VLCtransmission. The method also includes rejecting the first VLCtransmission, upon a determination that the first preamble does notmatch an expected preamble. The method further includes receiving asecond preamble associated with a second VLC transmission. The methodstill further includes synchronizing to the second VLC transmission,upon a determination that the second preamble matches the expectedpreamble.

Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, itmay be advantageous to set forth definitions of certain words andphrases used throughout this patent document: the terms “include” and“comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or,” is inclusive, meaning and/or; the phrases“associated with” and “associated therewith,” as well as derivativesthereof, may mean to include, be included within, interconnect with,contain, be contained within, connect to or with, couple to or with, becommunicable with, cooperate with, interleave, juxtapose, be proximateto, be bound to or with, have, have a property of, or the like; and theterm “controller” means any device, system or part thereof that controlsat least one operation, such a device may be implemented in hardware,firmware or software, or some combination of at least two of the same.It should be noted that the functionality associated with any particularcontroller may be centralized or distributed, whether locally orremotely. Definitions for certain words and phrases are providedthroughout this patent document, those of ordinary skill in the artshould understand that in many, if not most instances, such definitionsapply to prior, as well as future uses of such defined words andphrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIG. 1 depicts the full electromagnetic frequency spectrum, and abreakout of the wavelengths occupied by visible light;

FIG. 2 depicts several exemplary communication methods that can beenabled by VLC;

FIG. 3 depicts exemplary topologies supported by VLC communication;

FIG. 4 depicts an example of color channel assignments for differentfrequencies of the VLC spectrum;

FIG. 5 depicts examples of light interference that may occur during VLCcommunication;

FIG. 6 illustrates a comparison of VLC communication using one preambleand VLC communication using multiple preambles;

FIG. 7 depicts a method for attaining a fast lock time for a clock anddata recovery (CDR) unit, according to embodiments of the presentdisclosure;

FIG. 8A depicts a two-part preamble field that may be used by thetransceiver to obtain optical clock synchronization and distinguishmultiple topologies with an incoming message, according to a preferredembodiment of the present disclosure;

FIG. 8B depicts a preamble field for use in a burst mode transmission,according to one embodiment of the present disclosure;

FIG. 9 depicts an example of VLC transmission having an extendedpreamble, according to one embodiment of the present disclosure;

FIG. 10 depicts an example of VLC transmission that uses a fast lockingpattern during idle and receiving blocks, according to one embodiment ofthe present disclosure;

FIG. 11 depicts a more detailed example of an extended preamble mode,according to one embodiment of the present disclosure;

FIG. 12 depicts an extended preamble comprising a concatenation ofmultiple preamble types for separation of different types of VLCinterference, according to one embodiment of the present disclosure; and

FIG. 13 depicts a method for generating an extended preamble byintegrating a fast locking pattern into base preamble patternrepetitions, according to embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 through 13, discussed below, and the various embodiments used todescribe the principles of the present disclosure in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the disclosure. Those skilled in the artwill understand that the principles of the present disclosure may beimplemented in any suitably arranged visible light communicationnetwork.

The following documents and standards descriptions are herebyincorporated into the present disclosure as if fully set forth herein:

IEEE 802.15.7, which may be accessed at the time of filing athttp://www.ieee802.org/15/pub/TG7.html;

ECMA TC-47, which may be accessed at the time of filing athttp://www.ecma-international.org/memento/TC47-M.htm;

TG7 Technical Considerations Document: IEEE 802.15-09-0564-01-0007.

VLC enables a wide range of topologies with diverse requirements. FIG. 2depicts several exemplary communication methods that can be enabled byVLC. FIG. 2( a) shows an example of peer-to-peer (P2P) communication. Inthis example, a mobile phone communicates with another mobile phoneusing VLC. FIG. 2(b) shows another example of P2P VLC communication fornear field communication (NFC). NFC may be used where the communicationdistance is very short (<30 cm or so). In NFC, very high data rates(>100 Mbps) can be attained. In the example shown, a mobile phonecommunicates with a laptop computer using VLC.

FIG. 2( c) shows an example of a visible LAN (VLAN) system utilizing astar topology. In VLAN, the infrastructure lighting system alsofunctions as an access point and enables LAN service to one or moredevices, such as a laptop or a mobile phone. FIG. 2( d) shows an exampleof a broadcast topology known as information broadcasting (IB). In an IBsystem, a display at a public location (e.g., a mall or museum) couldbroadcast information (e.g., information about facilities, directions,or services) using VLC. Devices (e.g., mobile phones) that are in rangeof the broadcast may then receive the information. FIG. 2( e) showsanother example of a broadcast topology known as vehicular broadcasting(VB). VB may be used, for example, for conveying safety or trafficinformation from traffic signals or from other cars. It will beunderstood that the VLC topologies and devices shown in FIG. 2 are forexample purposes only. Other VLC topologies and devices are possible.

FIG. 3 depicts exemplary topologies supported by VLC communication. Forexample, VLC supports peer-to-peer topology (shown in FIG. 3( a)). VLCcommunication using peer-to-peer topology include near fieldcommunication (NFC). VLC also supports star topology (shown in FIG. 3(b)). An example of VLC communication using star topology is visible LAN(VLAN). VLC also supports broadcast topology (shown in FIG. 3( c)).Examples of VLC communication using broadcast topology includeinformation broadcasting (IB) and vehicular broadcasting (VB). VLC alsosupports visibility transmissions during idle or receive periods in theabsence of data communication to keep the light source with constantvisibility without flickering (shown in FIG. 3( d)). Thus,bi-directional, multicasting, and broadcasting capabilities may besupported using VLC communication.

A piconet is formed when at least two devices, such as a laptop and cellphone, connect. When a piconet is formed, one device can act as a masterand the other device (or devices) can act as slaves for the duration ofthe connection. A piconet may include only two devices (e.g., apeer-to-peer topology), or a piconet may consist of multiple devicesconnected to a single master (e.g., a star topology). It should beunderstood that while certain embodiments of the present disclosure aredescribed with reference to piconets, such a reference is for examplepurposes only, and should not be construed to limit the disclosure tosuch a network.

FIG. 4 depicts an example of color channel assignments for differentfrequencies of the VLC spectrum. As can be seen from the example, thevisible light spectrum may be divided into multiple color channels inorder to allow VLC operation in parallel channels. It is possible thatthere may be leakage from adjacent color bands into the band of thechosen color, causing adjacent color interference. Because the frequencyresponse at the photo-detector (receiver) can be quite broad (sometimescovering the entire visible range spectrum), care must be taken at thereceiver for separating multiple color channels; otherwise, the channelscould interfere with each other.

FIG. 5 depicts examples of light interference that may occur during VLCcommunication. In addition to interference from ambient light sourcessuch as sunlight, incandescent and fluorescent lamps, VLC may experiencethree types of network interference: Inter-topology interference (samecolor), intra-topology interference (same color), and adjacent colorinterference (different colors).

FIG. 5( a) shows an example of inter-topology interference that mayoccur when two VLC devices 502 and 504 are communicating in a P2P modebut experience interference from an infrastructure access point 506 thatis also using the same color channel. FIG. 5( b) shows an example ofintra-topology interference that may occur when a device 502communicating with one access point 508 in a VLAN network getsinterference from a second access point 510 nearby. Intra-topologyinterference is also applicable in vehicular topologies, for example,when a car gets interference from other cars or from multiple trafficlights. FIGS. 5( c) and 5(d) show the same physical arrangements asFIGS. 5( a) and 5(b), but the interfering light sources in FIGS. 5( c)and 5(d) operate in different (e.g., adjacent) color bands or channels.Since photodiodes can have a very broad frequency response, it ispossible that they may pick up signals in adjacent color bands. This maycontribute to interference unless very sharp filters are used. However,such filters may be expensive or complex to build.

Directionality is both an advantage and disadvantage of VLC. If a deviceexperiences interference, it can be difficult for that device to tellother devices of the interference. For example, looking at FIG. 5( a),infrastructure access point 506 may not aware of communication betweenP2P devices 502 and 504 since the P2P devices are not pointed towardsaccess point 506 and they may have limited transmit power. Thus,directional behavior can help with interference management (by beingdirectional) but also can be an issue (by not being able to resolveinterference due to directionality).

FIG. 6 illustrates a comparison of VLC communication using one preambleand VLC communication using multiple preambles. FIG. 6( a) depicts anexample of VLC communication using only a single preamble. In thisexample, a receiver 606 synchronizes to a preamble from an unwantedtransmitter 602. Receiver 606 then tries to decode the header and datafrom unwanted transmitter 602 and finally rejects it at the MAC layer,but only after receiver 606 determines it was not the intendedrecipient. In the meantime, a desired transmitter 604 attempts tocommunicate with receiver 606. However, since receiver 606 is busydecoding the transmission from unwanted transmitter 602, it does notlisten for desired transmitter 604's preamble. As a result, receiver 606completely misses the preamble from desired transmitter 604. Thus,having a single preamble leads to both wasted effort in decoding framesbelonging to unwanted transmitters and missing frames of a desiredtransmitter.

In contrast, FIG. 6( b) depicts an example of VLC communication usingmultiple preamble support, in accordance with embodiments of the presentdisclosure. In this example, receiver 612 listens for a transmissionfrom desired transmitter 610 having preamble 2. First, receiver 612receives preamble 1 from unwanted transmitter 608. Since preamble 1 isthe wrong preamble, receiver 612 rejects preamble 1 and ignores theremaining transmission from unwanted transmitter 608. Receiver 608continues to listen until it receives preamble 2 from desiredtransmitter 610. Once receiver 612 receives preamble 2, it thensynchronizes to the transmission from desired transmitter 610.

As illustrated in FIG. 6, since different preambles may be used todistinguish different topologies, adjacent LAN networks and piconets,and/or adjacent color channels, a receiver can reject preambles fromunwanted transmitters and continue listening until it sees the preambleof the desired transmitter, thus saving power and having betterprobability of acquisition.

In best cases, preambles are designed such that the operatingsignal-to-noise ratio (SNR) is less than that for the lowest data rate.There should be a low probability of misdetection, since the header orpayload cannot be decoded without synchronization. There also should bea low probability of false alarm, since it can cause the receiver tocontinuously trigger, thereby missing the useful data.

In wireless communication standards, multiple preambles are primarilyused to separate different logical channels. The preamble designs aretypically topology-independent. Since wireless communication systems arefairly omnidirectional (compared to VLC), it is easy to locate and talkto all devices within range and different devices can work withoutinterference. Traditional wireless systems also are built for aparticular topology in a particular location. For example, differenttechnologies may be used for VLAN (e.g., IEEE 802.11), P2P (e.g.,Bluetooth), and vehicular topologies (e.g., IEEE 802.11p); the differenttechnologies use different parts of the frequency spectrum and canco-exist. VLC provides support for multiple topologies within the sameenvironment but also permits use of the same spectrum across multipletopologies due to directionality. Many optical standards, such as thosepromulgated by Infrared Data Association (IrDA), use only a singlepreamble since they do not service as many topologies as VLC.

Thus, a need exists in VLC for inter-topology, intra-topology, andadjacent color separation with the help of multiple preamble support.Embodiments of the present disclosure provide this separation in thepreamble design and multiple preamble support. In certain embodiments,different types of VLC interference are distinguished using multiplepreambles and their properties. Inter-topology separation,intra-topology separation, and adjacent color separation based onmultiple preambles are provided. Although independent preambles could beconstructed to provide separation, providing independent preambles toensure separation for every different type could require hundreds orthousands of preambles, making the system more complex and increasingthe length of preamble considerably. Different embodiments modify thenumber of preambles, their sign, and their cover sequences, thuspermitting multiple repetitions of the same preamble to provideseparation between different types of VLC interference. Theseembodiments are summarized in Table 1 below.

TABLE 1 Methods for Providing Separation in VLC Type of InterferenceMethods for Separation Inter-topology interference Multiple preambles(same color) Intra-topology interference Sign change of preambles (samecolor) Adjacent color interference Cover sequence for preamble(different colors) repetitions

The method of separation associated with each type of interferencereflects a preferred embodiment of the present disclosure. However, itis noted that different embodiments could be arranged or combined sothat any combination could be used with any of the three described typesof interference (or any other type of interference) for VLC.

In an advantageous embodiment of the present disclosure, multiplepreambles are used to separate different VLC topologies and/or piconets.This allows a VLC receiver to synchronize to the correct transmitterand/or piconet and not connect to the wrong transmitter, thus wastingtime and power decoding unintended frames. This also allows the receiverto reject unwanted associations and quickly connect to the desiredtopology without missing the intended transmissions. As shown in FIG. 2,there are four main VLC topologies described in IEEE (i.e., P2P, VLAN,IB and VB) and one topology being considered for ECMA (i.e., NFC). Useof multiple preambles allows a VLC receiver to distinguish different VLCtopologies. For example, a VLC device that is built only for vehiculartopologies can automatically reject any transmissions from a deviceintended for P2P or NFC topologies based on the choice and use ofdifferent preambles.

In order to determine the number of possible preambles and the preamblelength, several factors are considered. Each preamble should be designedsuch that the operating SNR for decoding the lowest data rate is high.Each preamble should have a low probability of misdetection, since thedata cannot be decoded unless the preamble is detected and the receiveddata is synchronized to the transmission time. Each preamble also shouldhave a low probability of false alarm, since false alarms may make thereceiver trigger continuously and the useful transmissions may bemissed. It is desirable to minimize the overall number of differentpreambles, since a large number of preambles can make preamble selectionand design very complex and can cause false alarms. A small number ofpreambles also permits the preambles to be shorter in length, providingbetter throughput efficiency.

FIG. 7 depicts a method for attaining a fast lock time for a clock anddata recovery (CDR) unit, according to embodiments of the presentdisclosure. It is sometimes beneficial to use a 101010 . . . pattern ina preamble to attain a faster lock time for a CDR unit. A 101010 . . .pattern (or 010101 . . . pattern) represents a maximum transitionpattern that helps the CDR circuit attain a faster lock. In advantageousembodiments, the maximum transition pattern is 64 bits of alternating“1” and “0” bits, although other maximum transition patterns arepossible.

In certain embodiments, the preamble combines a fast locking patternwith multiple topology dependent preamble patterns in order to attainfast locking time and provide separation for multiple topologies andrejection of unwanted topologies. FIG. 7( a) depicts ‘z’ repetitions ofa topology dependent preamble pattern. The ‘z’ repetitions may includetime for locking. In contrast, FIG. 7( b) depicts ‘x’ repetitions of afast locking pattern, followed by ‘y’ repetitions of the topologydependent preamble pattern. The ‘z’ preamble pattern repetitionsrequired to achieve a lock in FIG. 7( a) may take a longer time than useof a maximum transition pattern (10101 . . . ). Thus, by using ‘x’repetitions of the maximum transition pattern and ‘y’ repetitions of thetopology dependent preamble pattern, as shown in FIG. 7( a), a lock maybe achieved faster than by using ‘z’ repetitions of just the preamblepattern.

In one particular example, ‘x’ may represent a 64-bit maximum transitionpattern, and ‘y’ repetitions of a topology dependent preamble patternmay include 4 repetitions of pattern P1, P2, P3, or P4 shown in Table 2above. However, it will be understood that other values of ‘x’ and ‘y’and other preamble patterns are possible.

FIG. 8A depicts a two-part preamble field that may be used by thetransceiver to obtain optical clock synchronization and distinguishmultiple topologies with an incoming message, according to a preferredembodiment of the present disclosure. The preamble field includes one ormore repetitions of a fast locking pattern (FLP) followed by one or morerepetitions of one of different topology dependent patterns (TDPs), forthe purpose of distinguishing different topologies. The MAC may selectan optical clock rate for communication during the clock rate selectionprocess. The preamble field may be sent at a clock rate chosen by thetransmitter and supported by the receiver. In certain embodiments, thepreamble field is a time domain sequence and does not have any channelcoding or line coding.

The preamble first starts with a FLP of variable length consisting of atleast 64 alternate 1's and 0's. The FLP is fixed to start as a “1010 . .. ” pattern (therefore, it ends with a ‘0’). This maximum transitionsequence (MTS) is used to lock the CDR circuit in the shortest time.Typically, the fast locking pattern length is chosen to not exceed themaximum shown in FIG. 8A. Before the CDR attains lock and recovers theclock, it has no way of determining the logic value of the transmittedsequence. After the fast locking pattern, 4 repetitions of one of fourTDPs are sent. The TDP is 15 bits in length. The TDP shall be invertedevery other repetition (represented in FIG. 5A as ˜TDP) to provide DCbalance and better synchronization.

FIG. 8B depicts a preamble field for use in a burst mode transmission,according to one embodiment of the present disclosure. For VLCcommunication between devices, it may be possible to send long streamsof data to the same destination even across multiple frames. In suchcases, a burst mode can be used that can reduce the preamble repetitionsand the interframe spacing (IFS) between the consecutive frames. Thereduced number of preamble repetitions improves the throughput of thesystem and eliminates the inefficiency of retraining the whole receiversince the previous frame was also sent to the receiver from the sametransmitter. For the burst mode transmission, the FLP is included onlyfor the first frame, such as shown in FIG. 8A. Subsequent frames do notinclude the FLP in the burst mode since it is already synchronized tothe transmitter. Thus, FIG. 8B depicts only the series of alternatingTDPs and inverted TDPs. This reduces the preamble length by at leasthalf and provides improved throughput at the MAC layer.

One advantageous embodiment employs four (4) TDP sequences to separatethe P2P, VLAN, IB and VB topologies. The TDP for the NFC topology may bedesigned independently, since NFC is used for very close communication(<30 cm) where the probability of interference is minimal. To determinethe four TDP sequences, one may search a Kasami short-sequence code-set.In general, a search of a 2^(n)-bit Kasami sequence may result in a TDPhaving 2^(n)-1 bits. In one example, four fifteen-bit TDP patterns P1-P4were obtained after a search for Kasami sequences of length sixteen(16). Table 2 shows the four TDP patterns that were obtained from thesearch.

TABLE 2 Four TDPs for Different VLC Topologies P1 1 1 1 1 0 1 0 1 1 0 01 0 0 0 P2 0 0 1 0 1 1 1 0 1 1 1 1 1 1 0 P3 1 0 0 1 1 0 0 0 0 0 1 0 0 11 P4 0 1 0 0 0 0 1 1 0 1 0 0 1 0 1

In one embodiment, the TDPs may be mapped to different topologies asshown in Table 3. It will be understood that any TDP pattern may be usedfor any topology. The mapping in Table 3 demonstrates only one specificexample.

TABLE 3 Mapping TDPs to Topologies Preamble Topology P1 Topologyindependent P2 Peer to Peer P3 Star P4 Broadcast

In one embodiment, the preamble patterns are inverted or “flipped”, andthe inverted patterns are used as additional preambles to distinguishtransmissions within a topology type (i.e., intra-topology separation).Here, an inverted pattern means that each bit in the pattern is changedfrom ‘1’ to ‘0’ or from ‘0’ to ‘1’. Table 4 shows the inverted TDPs thatare based on the preamble patterns of Table 2.

TABLE 4 Inverted Topology Dependent Patterns (TDP) for Distinguishingwithin a Topology Type −P1 0 0 0 0 1 0 1 0 0 1 1 0 1 1 1 −P2 1 1 0 1 0 00 1 0 0 0 0 0 0 1 ~P3 0 1 1 0 0 1 1 1 1 1 0 1 1 0 0 ~P4 1 0 1 1 1 1 0 01 0 1 1 0 1 0

In typical wireless systems, inverted preamble patterns areindistinguishable from unflipped preamble patterns since thecommunication channel can provide a 180 degree phase shift. However, VLCsystems typically use energy-based detection with on-off keyingmodulation. Thus, by looking at the sign of the received correlation, adetermination can be made as to whether a TDP or its inverted patternwas transmitted. Thus, the same correlator can be used for detectingboth the preamble pattern and its inverse. Both patterns for a topologycan be searched simultaneously without the need for separate preamblepatterns.

This method can distinguish between two patterns within a topology type.Two patterns are sufficient in VLC, because in most cases, a single VLCdevice would not get interference from multiple devices of the samecolor and same topology due to the directionality of VLC. Higher densitycan be achieved with multiple color choices, which can be separatedusing adjacent color separation.

It is possible to derive and define more TDPs using the approachdescribed in the earlier embodiments for distinguishing topologies as aviable alternative for supporting more patterns within a topology type.The tradeoffs may include a longer preamble length and an increasedsearch time for locating a particular piconet or transmitter of aparticular topology.

Table 5 shows how both the TDP and the inverted version of the TDP canbe used for defining intra-topology separation. The table provides twopatterns to distinguish within a topology. If more patterns are desired,other methods, such as generating more preamble sequences or generatingmore cover sequences (described below), may be used.

TABLE 5 Mapping TDPs to Topologies TDP Topology P1, ~P1 Topologyindependent P2, ~P2 Peer to Peer P3, ~P3 Star P4, ~P4 Broadcast

In another embodiment, cover sequences are defined to separate adjacentcolor interference. An algorithm is provided to derive cover sequencesto provide this separation. Specific cover sequence patterns are alsoprovided. Since a preamble is repeated multiple times forsynchronization during a transmission, the preamble may be invertedaccording to a certain pattern to distinguish the pattern from otherpatterns. For example, when the next number in the pattern equals zero,then preamble P1 is transmitted. When the next number in the patternequals one, then preamble ˜P1 is transmitted. The pattern defining theorder in which the preamble is flipped is a cover sequence. The coversequence may also serve as a marker of the end of preamble transmission.For example, when a receiver encounters the preamble associated with thelast element of a cover sequence, the receiver knows that the preambletransmission has ended and the control and data are going to follow fromthe next transmitted symbol. Each logical channel (e.g., the channelsshown in FIG. 4) may be assigned a cover sequence.

Table 6 below lists seven (7) cover sequences that provide adjacentcolor channel separation for seven (7) logical color channels, such asthe color channels shown in FIG. 4. Each cover sequence has a length ofeight (8), which imposes a minimum requirement of eight (8) preamblerepetitions for VLC. In certain embodiments, these cover sequences areapplied only at the end of the preamble repetition pattern. For example,if the preamble is being repeated 32 times, the cover sequence is onlyapplied on the last eight repetitions. While Table 6 provides seven (7)cover sequences with a length of eight (8), it will be understood thatadditional cover sequences, including those having lengths other thaneight (8), are possible.

TABLE 6 Cover sequences for Distinguishing Adjacent Color InterferenceCover Sequence Distances Sum weight C1: 0 0 0 1 0 1 0 1 5 1 4 1 2 1 1 157.79 C2: 0 0 1 0 0 1 0 1 5 3 1 3 1 1 1 15 8.09 C3: 0 0 1 0 1 0 0 1 5 3 23 0 1 1 15 8.23 C4: 0 0 1 0 1 0 1 0 6 1 4 1 2 1 0 15 8.65 C5: 0 1 0 0 10 1 0 6 2 2 3 0 2 0 15 8.75 C6: 0 1 0 1 0 0 1 0 6 2 2 3 0 2 0 15 8.75C7: 0 1 0 1 0 1 0 1 7 0 5 0 3 0 1 16 9.41

According to one embodiment, an algorithm to generate cover sequences isbased on a weighted metric. Frame sync detection is typically based onthe sign change. For example, let x(n), n=0, . . . , 15 denote the coversequence. As a specific example,x(n)=++++++++ ++−−−++−x(n−1)=X+++++++ ++++−−+.

Compute y(n)=x(n) x(n−1) (also referred to as “differentialcorrelation”).

For the example shown above, y(n)=++++++ ++−−+−−−.

If the received sign pattern matches −−+−−−, then frame sync isobtained.

The distance metric of the cover sequence depends on the number ofmismatches. Higher distance means less probability of false frame sync.Distances closer to the end of the sequence are more important, sincethe decision at the end of the frame sync sequence is crucial todetermining frame detection and the initial parts of the sequence canget lost in AGC and frame detection algorithms. Hence, we can impose alinear weight on the distances as they go closer towards the end of theframe sync sequence.

The cover sequences provided in Table 6 are determined based on thisalgorithm. It will be understood that use of this or other algorithmsmay provide additional cover sequences. Such additional cover sequencesare within the scope of this disclosure.

The embodiments described above provide a simple mechanism to separateinter-topology, intra-topology and adjacent color interference usingonly four (4) preambles and seven (7) cover sequences. A brute forceapproach to generate a separate preamble for each interference typewould require 2^(4*2*7)=2⁵⁶ different preambles. A requirement for thismany preambles would result in a much larger preamble length, whichwould be impractical.

FIG. 9 depicts an example of VLC transmission having an extendedpreamble, according to one embodiment of the present disclosure. In thisembodiment, a preamble is extended in advance of a frame transmission.The extended preamble provides reduced flickering and improvedvisibility, while simultaneously offering additional time forsynchronization and additional receiver training for fast association.This can be very beneficial for topologies such as VB, where there maybe insufficient preamble length due to mobility, communication over longdistances, and/or a short communication timeline.

FIG. 9( a) shows a regular transmission of data using a preamble blockand a control block followed by a data block. Between transmissions,there are idle or receive blocks where the VLC transmitter does nottransmit anything. In contrast, FIG. 9( b) shows that each preamble isextended across the adjacent idle block. Thus, in FIG. 9( b), thetransmitter transmits the preamble for an extended time. The extendedpreamble provides improved visibility, reduced flicker, and fastersynchronization with the receiver. The use of these additional preamblerepetitions for visibility support may be referred to as extendedpreamble mode. In advantageous embodiments, the extended preamblerepetitions in the extended preamble mode do not use a cover sequence.Instead, the regular preamble has the end of frame provided by the coversequence pattern. However, it will be understood that other embodimentsmay permit use of a cover sequence in the extended preamble repetitions.

FIG. 10 depicts an example of VLC transmission that uses a fast lockingpattern during idle and receiving blocks in the extended preamble mode,according to one embodiment of the present disclosure. In thisembodiment, the fast locking pattern (e.g., 10101010 . . . ) may betransmitted in idle or receive periods to help provide reducedflickering and improved visibility, while simultaneously providingfaster and better synchronization.

In certain embodiments, the MAC data frame is sent to the PHY layer andbecomes the PHY payload, also called the physical-layer-service dataunit (PSDU). The PSDU is prefixed with a preamble sequence and a PHYheader (PHR) containing the length of the PSDU in octets. The preamblesequence enables the receiver to achieve symbol synchronization.

FIG. 10( a) shows a regular transmission of data using a preamble blockand a PHR block followed by a PSDU block. Between transmissions, thereare idle or receive blocks where the VLC transmitter does not transmitanything. In contrast, FIG. 10( b) shows that a FLP is transmittedduring each idle/receive block. The length of the idle/receive block maynot be an integral multiple of the length of the FLP. In such cases, itis acceptable to truncate the FLP and transmit a fraction of the FLP inorder to maintain visibility. The FLP provides improved visibility,reduced flicker, and faster synchronization with the receiver.

In another embodiment, a number of repetitions (e.g., four repetitions)of a FLP are followed by a number of repetitions (e.g., fourrepetitions) of the TDP. The repeated patterns are transmitted duringeach idle/receive block. In other embodiments, the number of repetitionsof the FLP and TDPs may be increased or decreased.

FIG. 11 depicts a more detailed example of an extended preamble mode,according to one embodiment of the present disclosure. In thisembodiment, an idle time block 1102 is followed by a period of repeatedpreambles 1104. The media access control (MAC) layer uses knowledge ofidle time block 1102 to determine how much idle time is present. The MAClayer then increases the number of preamble repetitions 1106 transmittedto cover idle time block 1102. The extended preamble is made contiguousto the existing preamble of the next frame transmission. The MAC layermay choose to either transmit a visibility pattern or an extendedpreamble in idle time block 1102. The choice is indicated to thephysical (PHY) layer via the MAC-PHY interface. For example, a variablesuch as “macUseExtendedPreamble” can be set in the MAC to indicate thechoice between the visibility pattern or an extended preambletransmission. In certain embodiments, the length of the idle time maynot be an integral multiple of the preamble length. In such cases, it isacceptable to transmit a fraction of the preamble (e.g., the latterpart) in order to maintain visibility. This fraction of the preamble maybe referred to as a truncated preamble.

FIG. 11 depicts an example of a truncated FLP preamble 1108. In FIG. 11,the preamble pattern is shown as pattern ‘1010’. Additional ‘1010’preamble blocks 1106 are repeated to fill idle time block 1102. Sinceidle time block 1102 is not an integral multiple of the FLP ‘1010’, afraction of the preamble pattern (e.g., ‘010’) may be sent to completethe idle time. Although FIG. 11 shows that truncated FLP preamble 1108is transmitted before the whole preambles in the extended preamble mode,it will be understood that truncated FLP preamble 1108 may betransmitted at other times, such as after the end of the frame.

FIG. 12 depicts an extended preamble comprising a concatenation ofmultiple preamble types for separation of different types of VLCinterference, according to one embodiment of the present disclosure. Theextended preamble is a multi-part preamble that separates different VLCinterference types (e.g., inter-topology separation, intra-topologyseparation, adjacent color separation) by concatenation of multiplepreambles, each designed to separate a type of VLC interference. In theembodiment shown, extended preamble 1200 includes three preamble blocks1202, 1204, and 1206. Preamble block 1202 includes preambles of PreambleType 1 configured to separate inter-topology interference. Preambleblock 1204 includes preambles of Preamble Type 2 configured to separateintra-topology interference. Preamble block 1206 includes preambles ofPreamble Type 3 configured to separate adjacent color interference. Adifferent preamble can be generated for each interference type based ona Kasami code set sequence search. The different preambles may beconcatenated to form a bigger extended preamble pattern that candistinguish all the different interference types.

As shown in FIG. 12, the length of each preamble type and the number ofpatterns in each type can be variable. For example, there may be ‘P’patterns of Preamble Type 1 in preamble block 1202, ‘Q’ patterns ofPreamble Type 2 in preamble block 1204, and ‘R’ patterns of PreambleType 3 in preamble block 1206. The cross-correlation between preambleswithin the same preamble type may be very good. Cross-correlationbetween preambles from different preamble types may not be as good.Hence, it will be important for the receiver to know the construction ofthis jointly coded preamble sequence so that it can stop the correlationat the right point when looking for the right type. For example, in thiscase, the receiver must first look only for Preamble Type 1 followed byPreamble Type 2 and Preamble Type 3. It will be understood that theordering of interference types shown in FIG. 12 (inter-topology,intra-topology, and adjacent channel separation) is only forillustrative purposes. Other orders of the interference types arepossible.

FIG. 13 depicts a method for generating an extended preamble byintegrating a fast locking pattern into base preamble patternrepetitions, according to embodiments of the present disclosure. Similarto FIG. 7( a), FIG. 13( a) depicts ‘p’ repetitions of a preamble havinglength ‘L’. In contrast, FIG. 13( b) shows a new preamble pattern thatappends a fast locking maximum transition sequence (MTS) (e.g., 10101 .. . or 01010 . . . ) to the start of each preamble. The new preamble hasa length ‘M’. However, only ‘q’ repetitions of the new preamble arerequired to achieve a lock. This make the overall locking time fasterthan the preamble pattern in FIG. 13( b).

In other embodiments, the maximum transition sequence can be integratedor added in a part of the preamble other than the start of the preamble(e.g., the middle or the end of the preamble). In other embodiments,other fast locking patterns (e.g., 100100100 . . . , 11001100 . . . ,etc.) may be used for locking instead of a maximum transition sequence.

VLC has been shown to be useful for a wide range of topologies. It isdesirable to not detect every VLC transmission since some VLC devicesmay be built or optimized for a single or subset of topologies. Fortopologies such as VB, time is of great importance since totalcommunication time between the devices may be small. Supporting multiplepreambles for VLC permits separation and rejection of interference fromdifferent topologies and enables connection to the desired piconet ortransmissions.

Although the present disclosure has been described with an exemplaryembodiment, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present disclosure encompasssuch changes and modifications as fall within the scope of the appendedclaims.

1. For use in visible light communication (VLC), a method for synchronization, the method comprising: transmitting a two-part preamble sequence, the preamble sequence comprising: one or more repetitions of a fast locking pattern (FLP) configured to be used for clock synchronization, and one or more repetitions of a topology dependent pattern (TDP) configured to be used to distinguish a plurality of VLC topologies.
 2. The method as set forth in claim 1, where the FLP is a maximum transition sequence (MTS) having a repeating 101010 . . . pattern.
 3. The method as set forth in claim 1, wherein the TDP is used to distinguish four VLC topologies, the topologies being: peer-to-peer, star, broadcast, and visibility.
 4. The method as set forth in claim 1, wherein the TDP is selected from a plurality of predetermined TDPs, each predetermined TDP associated with a Kasami sequence.
 5. The method as set forth in claim 4, wherein the plurality of predetermined TDPs comprise the 15-bit patterns “111101011001000”, “001011101111110”, “100110000010011”, and “010000110100101”.
 6. The method as set forth in claim 1, wherein at least one repetition of the one or more repetitions of the TDP is inverted for better synchronization performance.
 7. The method as set forth in claim 6, wherein every other repetition of the one or more repetitions of the TDP is inverted.
 8. The method as set forth in claim 1, wherein the two-part preamble sequence is transmitted in a first frame, the method further comprising: transmitting a preamble sequence without an FLP in a second frame in a burst mode.
 9. The method as set forth in claim 1, wherein the length of the FLP is variable.
 10. The method as set forth in claim 9, wherein the length of the FLP is between 64 and 16384 bits.
 11. The method as set forth in claim 1, where the TDP is repeated 4 times for determining the topology.
 12. The method as set forth in claim 1, where the preamble sequence is transmitted using an On-Off Keying modulation.
 13. For use in visible light communication (VLC), a method for transmitting an extended preamble, the method comprising: generating an extended preamble; and transmitting the extended preamble during a receive or idle mode for maintaining visibility support and for better synchronization performance.
 14. The method as set forth in claim 13, wherein the extended preamble is a fast locking pattern (FLP).
 15. The method as set forth in claim 13, wherein transmitting the extended preamble comprises transmitting a preamble a predetermined number of times during an idle or receive block.
 16. The method as set forth in claim 13, wherein a media access control (MAC) layer determines the extended preamble for transmission and indicates the determined extended preamble to the physical layer (PHY) using a variable in the MAC-PHY interface.
 17. The method as set forth in claim 13, wherein a media access control (MAC) layer determines the predetermined number of times the preamble is transmitted based on a length of the idle or receive block.
 18. The method as set forth in claim 13, wherein the extended preamble is truncated during transmission when the idle time is not an integral multiple of the preamble length.
 19. For use in visible light communication (VLC), a method for distinguishing multiple VLC transmissions, the method comprising: receiving a first preamble associated with a first VLC transmission; upon a determination that the first preamble does not match an expected preamble, rejecting the first VLC transmission; receiving a second preamble associated with a second VLC transmission; and upon a determination that the second preamble matches the expected preamble, synchronizing to the second VLC transmission.
 20. The method as set forth in claim 19, wherein the first VLC topology and the second VLC topology are different topologies.
 21. The method as set forth in claim 20, wherein the first VLC topology comprises communication in a first color channel and the second VLC topology comprises communication in a second color channel.
 22. The method as set forth in claim 19, wherein the first VLC topology and the second VLC topology are the same topology.
 23. The method as set forth in claim 19, wherein the first preamble is associated with a first cover sequence and the second preamble is associated with a second cover sequence. 